1. Field of the Invention
The present invention relates to improvements in apparatus for aspirating liquids, such as biological specimens, from containers for processing. The invention is particularly useful in automated hematology instruments for extracting blood samples from sealed test tubes and the like for analysis.
2. The Prior Art
In conducting tests on biological liquids, it is common to employ automated instruments to extract liquid specimens from sealed containers, e.g., test tubes and the like. Such instruments typically include a movably-mounted sample aspiration probe that is adapted to puncture a rubber stopper atop a stationary specimen container to access and aspirate the liquid therein. These instruments often include a mechanism for sensing the presence of a specimen container at a desired specimen-aspiration position before the probe is moved in a direction to expose the probe tip, and most instruments include a device for stripping the rubber stopper from the aspirating probe as the probe is withdrawn from the container to prevent the stopper (and the container to which it is sealed) from following the movement of the probe due to frictional forces between the probe and stopper.
An automated instrument of the above type is disclosed in U.S. Pat. No. 5,517,867 to Ely et al. The disclosed apparatus is adapted to move a liquid-aspiration probe along a vertical axis into and out of a stoppered test tube containing a liquid of interest. The aspiration probe is supported in a vertical orientation above the test tube by a rigid, horizontally-extending arm that forms part of a frame. Such arm is raised and lowered by lead screw that is incrementally rotated by a motor. As the lead screw rotates in a first direction, the probe-supporting arm is lowered to a position in which the tip of the aspiration probe punctures a rubber stopper that seals the liquid to be aspirated within the tube. Further rotation of the lead screw in the same direction serves to position the probe tip in an aspiration position below the liquid level in the tube. Following liquid aspiration, the stepper motor operates to rotate the lead screw in the reverse direction, thereby raising the probe-supporting arm to a position in which the probe tip clears the top of the tube. To strip the rubber stopper from the aspiration probe during withdrawal of the probe from the tube, a downward force is applied to the stopper during upward movement of the probe. Such force is provided by a relatively massive second arm that extends horizontally, i.e., parallel to the probe-supporting first arm, and hangs downwardly from the first arm in a positioned intermediate the bottom of the first arm and the top of the test tube. The bottom surface of the second arm supports a “foot” that serves, during the downward movement of the first arm, to both guide the vertical travel of the aspiration probe, and to detect and position the underlying test tube so that the center of the rubber stopper is aligned with the path of the probe tip.
In liquid aspirating instruments of the above type wherein the tip of an aspiration probe is used to puncture a rubber stopper that seals the liquid in the container, the friction between the external surface of the probe and the rubber stopper can easily exceed 10-12 pounds. Considering that there can be additional frictional forces at work in guiding the downward movement of a weighted plate used to apply a stripping force to the tube stopper in instruments of the above-described type, the required weight of such plate may be as much as 18-20 pounds to assure that the stripper mechanism operates reliably. Thus, it will be appreciated that this approach is disadvantageous if only from the standpoint that it adds considerable weight to the instrument.
In U.S. Pat. No. 5,935,523 to McCandless, another automated apparatus is disclosed for accessing a liquid in a sealed container. This apparatus addresses the stripping problem noted above and provides several different solutions. First, to reduce the frictional force between the rubber seal on the container and a penetrating aspiration probe, an independent cap-piercing member is provided for cutting a slit in the cap (stopper) of a container (a test tube) before the aspiration probe enters the tube through the stopper. The slit, of course, facilitates movement of the probe through the stopper by reducing the frictional forces between the two. Further, means are provided for lubricating the cap-piercing member before it slices through the cap, and a residue of lubricant on the stopper will further reduce the stripping force required to separate the aspiration probe from the probe following aspiration. Finally, a “foot” which presses on the stopper during aspiration is mechanically locked in place by a solenoid-operated latch that operates during the liquid-aspiration and probe removal procedures. Thus, the use of a weighted foot, as taught by the above reference, is obviated through the combination of these relatively extreme features. While the aspirating apparatus of McCandless et al. does, indeed, facilitate the stripping function, it does so at the cost of a relatively complex mechanism. Further, the apparatus may be viewed as problematic in that it requires repositioning of the entire liquid aspiration apparatus after cap-piercing has been effected to align the aspiration probe with the container axis, and it further requires substantially perfect parallel alignment between two independently movable shafts, i.e., the aspiration probe and the cap-piercing member. The latter can be particularly problematic since any binding friction caused by the misalignment between the two shafts can produce a malfunction of the apparatus.